The invention relates to semiconductor circuits (ICs) and more particularly to a circuit used to provide magneto resistive biasing voltage with fast recovery.
Magneto resistive sensors are used to detect magnetic signals in various applications such as gauss probes, global positioning systems and magnetic storage hard drives. Magnetic storage hard drives use a magnetic coil to write signals onto magnetic media and use magneto resistive sensors to read signals from the magnetic media. The magneto-resistive sensors are current biased to maximize performance.
FIG. 1 is an example of a circuit used to bias the current for a magneto resistive sensor (MR) used in hard drives. The current bias circuit of FIG. 1 includes an external magneto-resistive sensor (RMR) 102, a reference digital to analog converter (REF_DAC) 104, an internal reference resistor (RMR_REF) 106 a resistor 108, a filter 110, a servo circuit 112, and a magneto resistive digital to analog converter (MR_DAC) 114. The REF_DAC 104 further includes a switch 116 and a resistor 118. Filter 110 is a capacitor. Servo circuit 112 further includes a comparator 120, a capacitor 122, a switch M2 124 and another capacitor 126. (MR_DAC) 114 further includes a set of switches 128A, 128B, and 128C arranged in series with a set of resistors 130A, 130B, and 130C such that the combination of switch 128A and resistor 130A, the combination of switch 128B and resistor 130B, and the combination of switch 128C and resistor 130C, are all arranged in parallel.
In FIG. 1 current is biased by first flowing a reference current IREF through the switch 116 which is grounded, and then through the resistor 118 and finally through the reference resistor 106 which is set to 50 ohm. The IREF generates a reference voltage (VREF) which in turn produces a low pass and heavily filtered gate voltage (VREF_G) at servo circuit 112 and switch M2 124 as well as a second reference voltage VREF_1. Since the MR_DAC 114 can be programmed according to the specifications of the system, the current at magneto-resistance sensor can be calculated. For example, if VREF=VREF_1 and MR_DAC are configured to match REF_DAC then the current at the magneto resistive sensor is IREF. However, if the magneto-resistance is not known, such as for example when the system is changing, then this circuit does not produce reliable results because it does not know the resistance. An example of when the system is changing is when the hard drive begins to spin up and the MR sensor transitions from an idle or sleep state to an active state.
Therefore, what is needed is a system and method for providing a fast recovery for voltage bias applied to an MR sensor, when a voltage is applied to the MR sensor very quickly while the MR sensor transitions from an idle or sleep state to an active state.